The present invention relates to methods and apparatuses for characterizing or evaluating the strength or effectiveness of oil-based drilling fluids for use in drilling well bores in subterranean formations. The invention relates particularly to methods and apparatuses for characterizing water-in-oil or invert emulsion fluids for use in drilling well bores in hydro-carbon bearing subterranean formations.
Drilling fluids are frequently used in oil and gas drilling operations. These fluids serve many purposes including, but not limited to, removing the cuttings produced, lubricating and cooling the drill bit, and supporting the walls of the hole being drilled.
Oil-based drilling fluids are frequently used when drilling wells for oil and gas. These oil-based drilling fluids are typically water-in-oil emulsions that are stabilized with the addition of emulsifiers. The water phase is usually about 5% to about 40% of the total liquid volume and is usually comprised of (but not limited to) calcium chloride brine. If the formulation of the invert emulsion drilling fluid becomes unbalanced, due to, for example (without limitation), contamination, improper product additions, or thermal degradation, the oil-based drilling fluids tend to revert to an oil-in-water emulsion. As a result of the reversion to an oil-in-water emulsion, the water becomes the continuous phase and can cause the solids in the drilling fluid to become water wet. Such wetting has significant negative consequences to a drilling operation and requires expensive remedial action in order to prevent the loss of the well.
An Electrical Stability Tester (EST), such as the Fann 23D available from Fann Instrument Company in Houston, Tex., is typically used to characterize invert emulsion oil-based drilling fluids. The probe of the Fann 23D EST is shown generally in FIG. 1. The EST probe consists of a connector, a high voltage cable, and a molded electrode arrangement (probe). The EST probe is assembled with two electrodes precisely spaced apart in a molded, electrically insulating material such as plastic. In order to characterize the stability of an oil-based drilling fluid, the probe is placed into a sample of the oil-based drilling fluid such that the fluid fills the gap between the electrodes. The oil-based drilling fluids are typically water-in-oil emulsions with oil being the continuous phase. Therefore, the oil-based drilling fluid acts as an insulator. An increasing AC voltage is applied to the oil-based drilling fluid sample, across the two electrodes, while the current flow between them is monitored. Once the potential difference between the electrodes reaches a certain level, the oil-based drilling fluid will break down and a conductive path will be formed between the two electrodes. The peak voltage required to cause the breakdown is defined as Electrical Stability of the oil-based drilling fluid. The American Petroleum Institute's “Recommended Practice Standard Procedure for Field Testing Oil-Based Drilling Fluids”, API Recommended Practice 13B-2, Third Edition, February 1998 (“the API Procedure”), is incorporated herein by reference. Paragraph 8.1.1 of the API Procedure defines the Electrical Stability of an oil-based drilling fluid as “the voltage in peak volts-measured when the current reaches 61 μA.” Frequent measurements of Electrical Stability must be obtained in order to monitor the strength and characteristics of the oil-based drilling fluid during the drilling operations. Typically a Mud Engineer in the field must manually carry out the necessary steps to obtain an Electrical Stability measurement.
However, the typical method of use of an EST, as discussed above, has many drawbacks. One disadvantage of the current method is that the oil-based drilling fluid sample must be screened with a Marsh funnel and heated or cooled to the test temperature prior to each measurement in order to obtain consistent Electrical Stability values. This can be time consuming and burdensome. Another shortcoming of the current method is that the probe must be held stationary in the sample during the measurements in order to obtain repeatable measurements.
Yet another problem with the current method is that the contamination on the surface of the molded insulation of the EST probe can create a conductive path around the electrode gap allowing the current flow between the electrodes to bypass the sample in the gap. In order to reduce the impact of the contaminants, the probe must be very carefully cleaned between the measurement of different samples in order to prevent the build up of insulating or conducting films on the surface of the probes. Currently, this cleaning is accomplished using ultrasonic cleaning, solvents or passing rags or paper towels through the electrode gap in order to mechanically clean the probe.
The above pitfalls associated with the current method of measuring Electrical Stability make this method time consuming and prone to errors. The problem is further compounded by the fact that typically a Mud Engineer must manually obtain frequent Electrical Stability measurements during the drilling operations thereby making the current methods also labor intensive and expensive.